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Communication: How Communication is Done
The basic method of communicating with spacecraft - in fact, the only viable
method - is by radio transmission, generally called "telecommunications."
Information of various kinds is digitized onboard the spacecraft and then
transmitted to the ground over one of several microwave frequency bands. On
the ground it is received by one of NASA's large tracking and communication
antennas, from which it is relayed to a data processing organization at one of
the NASA centers. This process of spacecraft-to-ground communication is
called "downlink." Information, such as command sequences,
must also be sent from the ground to the spacecraft, and this process is
called "uplink." The signals in both directions are referred to
as "telemetry."
In the case of TOPEX/Poseidon, the communication link is a bit more
complicated. Instead of direct communication between the ground and the
satellite, the TOPEX/Poseidon mission utilizes NASA's Tracking and Data Relay
Satellite System, or TDRSS. This is a constellation of six satellites that were
launched in the late 1980s and early '90s into geostationary orbits 35,880
kilometers (22,300 miles) above the Earth. (By "geostationary"
we mean that the satellites' orbits are synchronized with the Earth's rotation,
so that they appear to "hang" over the same spot on Earth all the
time.) One or more of the TDRSS satellites are in view of the TOPEX/Poseidon
spacecraft at almost all times. The spacecraft communicates with the TDRSS
satellites using its movable high-gain dish antenna.
The other component of the TDRSS is the ground station at White Sands, New
Mexico, which sends and receives signals to and from the system's satellites.
From the White Sands Ground Terminal the TOPEX/Poseidon signals are
relayed via a Domestic Communications Satellite (DOMSAT) to the NASA
Goddard Space Flight Center (GSFC) at Greenbelt, Maryland. GSFC performs
some processing on data related to the spacecraft orbit, then relays the data
stream by satellite and ground link to the Project Operations Control Center
(POCC) at JPL in Pasadena, California.
The TDRSS link provides many benefits to TOPEX/Poseidon, which was the
first JPL mission to use the system for communications. With TDRSS, the mission
has the ability to conduct "real-time" communications with the
satellite for over 20 out of 24 hours. By contrast, JPL's traditional method of
using the antennas of the worldwide Deep Space Network to communicate with
spacecraft allows for no more than about 3-1/2 hours of real-time
communication out of every 24. In addition, TDRSS offers tracking and
communications for as many as 32 satellites simultaneously, whereas the
existing ground station network can service no more than two satellites at a
time. Thus, there is less chance that TOPEX/Poseidon communications will be
interrupted by higher-priority transmissions. Finally, signal switching operations
on the TDRSS satellites are very fast - they can take place at 300 million data
bits per second, which is the equivalent of about 20 volumes of an
encyclopedia in one second!
The onboard communications hardware on TOPEX/Poseidon includes the
high-gain antenna (shown here) for communication with the TDRSS satellites, a
transmitter, a receiver, and three tape recorders for storing data. Because the
tape recorders are so important to data recovery, they are rotated in service to
reduce the wear on each unit as much as possible. Specifically, each one
records for 8 hours plus a few minutes of overlap each day. Playbacks of the
data (i.e., transmissions to the ground) are pre-scheduled, but real-time
commands from the ground are used to configure the recorders and to initiate
the playbacks, or "data dumps."
There is also some backup hardware on the spacecraft, in case the main
communications link should fail. For example, on both the top and bottom of
the spacecraft are lower-gain "omni" antennas (i.e., antennas
that radiate and receive in all directions) that can be used to transmit
telemetry if the high-gain antenna malfunctions. The upper omni antenna
(show here,center) can be used to transmit and receive via the TDRSS. The
lower omni can be used to communicate through the ground antennas of the
Deep Space Network (DSN), NASA's primary tracking and communications system.
The DSN has telecommunications sites in Spain, Australia, and the Mojave
Desert in California, one of which is always in sight of an orbiting spacecraft.
(Note: during the mission, the omni antennas have sometimes been used even
though the high-gain antenna has not, as yet, malfunctioned.)
Power for communication on the spacecraft is provided by a large solar
"paddle" or array (shown here under construction), except when
the spacecraft is in the shadow of the Earth, a situation known as
"occultation." During these periods, power is provided by batteries,
which are then recharged when the solar array comes back on line. The solar
array is the largest and one of the most delicate pieces of spacecraft equipment.
During launch, the array was folded along the Instrument Module. After
deployment it expanded to 8.89 meters long and 3.30 meters wide, with a
maximum power output of 3385 watts.
The final element of the communications system on the spacecraft is the
onboard computer, which is the heart or, more appropriately, the mind of what
is called the Telecommunications and Data Handling Subsystem. The computer
receives and stores commands from the ground (either sequences sent ahead
of time or commands in real time) and executes them at the proper moment.
It also operates the science instruments and the tape recorders and monitors
the health and status of the spacecraft.
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